1. Field of the Invention
The present invention relates generally to a semiconductor integrated circuit (IC) device. More particularly, the invention relates to a semiconductor IC device adapted to selectively perform a lock-out operation according to an operating mode of the integrated circuit device.
2. Description of the Related Art
Significant effort has been made to develop electronic devices with low operating voltages. Benefits of low-voltage electronic devices include, for example, decreased heat generation and decreased power consumption. These benefits are particularly important in portable electronic devices having limited power resources and in devices susceptible to heat damage.
A number of design issues arise when creating low-power electronic devices. One design issues relates to the way the low-voltage devices interact with other devices. For example, an issue arising due to the interaction of portable electronic devices with removable memory cards is described below.
Portable electronic devices often provide capabilities requiring relatively large amounts of memory. For instance, devices such as mobile phones and personal digital assistants (PDAs) often contain digital camera equipment producing large datasets in the form of still images and video and requiring a significant amount of memory for storage and processing.
One type of memory that is commonly used to store and process data in portable electronic devices is NAND flash memory. A common of providing NAND flash memory to a portable electronic device is in the form of a portable storage unit such as a removable memory card. A NAND flash memory chip is generally embedded in the removable memory card, and the flash memory chip becomes operatively connected with the device when the memory card is coupled to the device, e.g., by inserting the card into a card reader slot on the device. Examples of removable memory cards containing embedded NAND flash memory chips include multimedia cards (MMC), secure digital (SD) cards, smart media cards, and Compact Flash cards. Each of these types of memory cards is used to store and process various types of data including audio, video, etc. In addition, each of these types of memory cards is commonly used in devices such as digital cameras, MP3 players, PDAs, gaming devices, facsimile machines, scanners, and printers.
A removable memory card is typically designed to operate under a specific set of conditions. For example, a removable memory card may be designed to operate with a power supply voltage of 3.3V. However, where a memory card designed to operate with a power supply voltage of 3.3V is coupled to a portable electronic device using a lower power supply voltage, the memory card may not work properly.
In order to prevent problems from arising due to a low power supply voltage in a portable electronic device, a lock-out circuit may be included in a portable storage unit to disconnect the portable storage unit from the low power supply voltage. The lock-out circuit cuts off the supply of an operating voltage VCC (or an external power supply voltage EVC) in the portable storage unit whenever the operating voltage VCC falls below a predetermined reference voltage level. Once operating voltage VCC is cut off from the portable storage unit, the lock-out circuit executes a recovery operation to restore operating voltage VCC to above the reference voltage level. A lock-out circuit and a system including the same are disclosed, for example, in U.S. Pat. No. 6,434,653.
In addition to removable card systems, the lock-out circuit can also be included in flash memory devices fabricated as a single chip, or in other semiconductor devices including flash memories. The lock-out circuit can also be included in other semiconductor memory devices besides flash memory devices.
FIG. 1 is a waveform timing diagram illustrating the operation of a conventional lock-out circuit in a semiconductor memory device. Referring to FIG. 1, the lock-out circuit switches a lock-out control signal VLKO to a logic level “high” when operating voltage VCC (or an external power supply voltage EVC) falls below a predetermined detection voltage level Vdetect (indicated by an arrow (1)).
Once lock-out control signal VLKO is switched to logic level “high”, operating voltage VCC is disconnected from the semiconductor memory device and a program or erase operation (PGM or ERS Execution) of the semiconductor memory device is terminated. Then, the lock-out circuit executes a recovery operation (PGM or ERS recovery) to restore operating voltage VCC to above detection voltage level Vdetect.
One problem with the method of operating the semiconductor memory device described above is that it disconnects operating voltage VCC from the semiconductor memory device in certain cases where the level of operating voltage VCC (or a low voltage of the external power supply voltage EVC) is sufficient to accomplish a specific function being performed by the semiconductor memory device. For example, where a function requiring a voltage level indicated by an arrow (2) is being performed by the semiconductor memory device, operating voltage VCC is disconnected and the function is terminated.